Tectonic Setting and Characteristics of Natural Fratures in Mesaverde And

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Tectonic setting and characteristics of natural fractures in Mesaverde and Dakota reservoirs of the San Juan Basin

John C. Lorenz and Scott P. Cooper, Geophysical Technology Department 6116, MS 0750, Sandia National Laboratories, Albuquerque, NM 87185-5800

Abstract The Cretaceous strata that fill the San Juan Basin of northwestern New Mexico and southwestern Colorado were shortened in a generally north-south to north northeast- south southwest direction during the Laramide orogeny. This shortening was the result of compression of the strata between southward indentation of the San Juan uplift at the north edge of the basin and northward to northeastward indentation of the Zuni uplift from the south. Right-lateral strike-slip motion was concentrated at the eastern and western margins of the basin to form the Hogback monocline and the Nacimiento uplift at the same time. Small amounts of shear may have occurred along pre-existing basement faults within the basin as well. Ver- tical extension fractures, striking north-south to north northeast-south southwest (parallel to the Laramide maximum horizontal compressive stress) with local variations, formed in both Mesaverde and Dakota sandstones under this system, and are found in outcrops and in the subsurface. The less-mature Mesaverde sandstones typically contain relatively long and irregular vertical extension FIGURE 1—Structure contour map of the inner or central section of the fractures, whereas the underlying quartzitic San Juan Basin. Contours are drawn on the top of the Upper Cretaceous Dakota sandstones contain more numerous, Pictured Cliffs Sandstone, which overlies the Mesaverde Group. The basin shorter, sub-parallel, closely spaced exten- is asymmetric with the northwest trending synclinal hinge near the north- sion fractures. Conjugate shear fractures in ern edge of the basin. several orientations are also present locally in Dakota strata. the entire basin (e.g., Kelley, 1957; Fassett, Location and general geology Introduction 1991; Laubach and Tremain, 1994) are pri- The San Juan Basin of northwestern New marily descriptive rather than attempting Mexico and southwestern Colorado is This paper has two objectives: 1) to pro- to interpret the kinematics of the important asymmetric, and has strata dipping gently vide a brief characterization of natural structures. We have re-examined the basic to the north and northeast toward the off- fractures that control the production of structural building blocks of the basin and center synclinal hinge located near the Col- natural gas from sandstone reservoirs in have constructed an unconventional but orado–New Mexico state line (Fig. 1). Mesaverde and Dakota strata in the San plausible tectonic scenario for the post- Most of the hydrocarbon production Juan Basin, and 2) to reconstruct a plausi- Jurassic structural history of the San Juan from the San Juan Basin comes from an ble tectonic framework for the genesis of Basin. Because the impetus behind this inner zone, approximately delineated by these fractures. The tectonic model pre- study was hydrocarbon reservoir charac- the outcrop belt of the Upper Cretaceous sented here provides a basis for the extrap- terization, our tectonic scenario was con- Pictured Cliffs Sandstone and separated olation of fracture characteristics from out- structed to provide a framework that from the peripheral areas by a relatively crops and core to the subsurface and for would explain the observed outcrop and continuous hogback nearly 400 km (154 the prediction of fracture characteristics in subsurface natural fracture characteristics mi) long, consisting of gently dipping to the subsurface. This paper is a shortened and that would permit a basic level of sub- near-vertical strata. As described below, version of the report containing more of surface fracture prediction. Thus the paper however, despite the apparent continuity the supporting data (Lorenz and Cooper, presented below is an inversion of the nor- of this feature, this hogback does not have 2001). Copies of the longer report can be mal procedure wherein fractures would be a common mechanical origin along its obtained directly from the authors or are used as kinematic indicators and as first- entire length. Lower Cretaceous strata crop available from the U.S. Department of level data in the reconstruction of a tecton- out at the margins of the San Juan Basin Commerce, National Technical Informa- ic history. Nevertheless, fracture evidence and extend into the subsurface to depths as tion Service, Springfield, VA, or online at and tectonic reconstruction are mutually great as 2,000 m (6,562 ft), where they have www.osti.gov/bridge. supporting, and converge on the same con- been penetrated by thousands of natural- Most of the published tectonic recon- clusion that would have resulted from a gas wells. structions of the San Juan Basin area focus more standard approach to the problem. Significant thicknesses of overburden on one or two of the structural elements strata were stripped from the surface of the that bound the basin. Those that examine basin in post-Cretaceous time. As much as

February 2003, Volume 25, Number 1 NEW MEXICO GEOLOGY 3 extension. Our work suggests this fracture set is part of the regional, early Laramide fracture system. Kelley (1957) and Kelley and Clinton (1960) note several domains of relatively uniform fracture strikes in the San Juan Basin as part of their broad-scale aerial- photo study of fractures on the Colorado Plateau. However, the scale of their study did not permit detailed examination and assessment of the fractures at ground level, and subsequent investigators have not recognized these domains. Previous subsurface studies infer the presence of fractures in Cretaceous San Juan Basin reservoirs based on several indirect criteria including association with faults recognized on seismic data (e.g., TerBest, 1997; DuChene, 1989) and anom- alous production rates (e.g., Emmendorfer, FIGURE 2—Extension and conjugate shear fractures as observed and created in laboratory compres- 1992; Gorham et al., 1979; London, 1972; sion tests (σ1—maximum principal stress; σ2 — intermediate principal stress; σ3—minimum princi- Ouenes et al., 1998). Fracturing is typically pal stress; modified from Weijermars, 1997). assumed to have been caused, or at least enhanced, by flexures. Actual subsurface fracture data are rare, although Ortega and 2,500 m (8,202 ft) of overburden may have these sections suggest locally significant Marrett (2000) publish subsurface fracture been removed from northern parts of the thrust indentation and overhang at the data for the Mesaverde for three wells in basin (Bond, 1984). The previous deep bur- basin margins. Erslev (2001) and Cather the central part of the basin indicating a ial and resulting high overburden stress (1999) interpret the structure of the dominate north-south-striking fracture set, accounts for horizontal stylolites that are Nacimiento uplift on the eastern margin of and Lorenz et al. (1999) present a prelimi- locally common in Dakota sandstone the San Juan Basin in the context of overall nary synthesis of the tectonic framework cores. Laramide kinematics. of the basin based on wellbore-image logs Previous studies of natural fractures and and measured fracture characteristics in 32 Fracture terminology coal cleats in the San Juan Basin are prima- Dakota cores. The interpretations herein For simplicity, breaks in rock will be rily descriptive and concentrate on out- build on the conclusions of Lorenz et al. described as either extension fractures or crops of Mesaverde strata (e.g., Whitehead, (1999), expanding them to encompass shear fractures within this paper, consis- 1997; Tremain et al., 1991; Laubach, 1992). Mesaverde reservoirs. tent with the terminology used within the These studies suggest a demarcation in petroleum industry. Extension fractures surface fracture domains that is approxi- Tectonic setting (also termed joints, or tensile fractures or mately coincident with the New Mexi- dilation fractures or Mode I fractures: Pol- co–Colorado border. This demarcation was The inner part of the San Juan Basin lard and Aydin, 1988) have displacements not apparent during the course of this appears to be relatively structureless, with perpendicular to the fracture surfaces. study, perhaps because this study concen- low-amplitude flexures and gentle region- Shear fractures have some displacement trated on the oldest fractures present in the al dips. However, the basin is underlain by parallel to the fracture surface (also termed outcrops, those most likely to be present in a Proterozoic crystalline basement with Mode II or Mode III fractures depending the subsurface rather than fractures related many ancient and locally reactivated faults on displacement relative to the fracture to folding and uplift. Whitehead (1997) trending northeast-southwest and north- front: Pollard and Aydin, 1988). Both even suggests that many of the fractures in west-southeast (Taylor and Huffman, extension and shear fractures can be outcrops, particularly on the Chaco slope 1998). These faults may have been impor- formed in laboratory compression tests area, were caused entirely by surficial, val- tant controls on the fracture patterns in the with specific orientations with respect to ley-wall gravitational processes. In addi- overlying strata, but we suggest here that the applied stress. Extension fractures tion, Condon (1997) reports that the aver- the tectonic features surrounding the basin form perpendicular to the least compres- age strike of face cleats in coals at the provided the main controls in forming the sive stress (σ3), parallel to maximum com- northern margin of the San Juan Basin is pervasive, regional fracture pattern. pressive stress (σ1), and bisect the acute not only oblique to fractures in associated The principal tectonic features bounding angle between the conjugate shear frac- sandstones but that the cleat and fracture the San Juan Basin are the San Juan uplift tures (Peng and Johnson, 1972; Long et al., strikes rotate in opposite directions along or dome to the north, the Zuni uplift to the 1997; Fig. 2). strike. Therefore the coal cleats in these south, the Defiance uplift and Hogback strata may have had a separate origin from monocline to the west, the Nacimiento Previous work the fractures in sandstones addressed here, uplift to the southeast, and the Archuleta Published structural studies associated accounting for some of the discrepancies in anticlinorium to the northeast (Fig. 3). with the San Juan Basin focus on the spe- orientation data between this study and These structures were largely formed dur- cific tectonic elements bounding the basin earlier studies. A multi-stage, multi-direc- ing the Laramide orogeny (Kirk and Con- (e.g., the Nacimiento uplift: Woodward, tional tectonic origin for faulting and frac- don, 1986), although many of them had 1987; Baltz, 1967), or deal with the basin as turing in this area is suggested by Ruf attained some structural expression during a sub-unit within the Colorado Plateau (2000) and Erslev (1997). Ruf (2000) earlier tectonic events, and some were (e.g., Chapin and Cather, 1981; Cather, describes a regional fracture set in the reactivated during Tertiary Rio Grande 1999). Taylor and Huffman (1988, 1998) northern San Juan Basin as striking from rifting (Kelley, 1957; Slack and Campbell, published several seismic sections across north northwest to north northeast but 1976; Woodward, 1987). the monoclines that bound the inner basin; relates their formation to post-Laramide

4 NEW MEXICO GEOLOGY February 2003, Volume 25, Number 1 basins throughout the rest of the Rocky Mountain region. Precambrian rocks within the San Juan uplift are at least 4,300 m (14,108 ft) above sea level creating approximately 6,100 m (20,013 ft) of structural relief between the top of the uplift and the deepest parts of the adjacent basin. Much of the difference (3,500 m; 11,482 ft) is taken up across the short distance spanned by the Hogback monocline where it marks the northern border with the San Juan Basin. Moreover, although poorly exposed, small east-west striking, southward-verging thrust faults have been mapped in the surface on the southern side of the San Juan uplift. Larg- er, south-vergent thrusts have been docu- mented in the subsurface along and south of the Hogback monocline (Taylor and Huffman, 1988, 1998; Huffman and Taylor, 1999). This suggests that the basin and uplift are tectonically/structurally linked. The fracture data presented below are, in fact, compatible with a model wherein the San Juan uplift was thrust southward into the San Juan Basin.

Eastern and western margins of the basin The Defiance uplift bounds much of the western side of the San Juan Basin and consists of a north-striking asymmetric block with the steepest limb forming the Defiance monocline on its eastern edge. The sinuosity of the monocline is due to several southeast-plunging anticlinal and synclinal cross-folds, giving the Defiance monocline an en echelon character and suggesting approximately 13 km (8 mi) of right-lateral wrench faulting at depth (Kel- ley, 1967; Woodward and Callender, 1977). FIGURE 3—Index map showing the structural elements of the San Juan Basin. Areas of steep dip (monoclines) are shown in green with the direction of dip indicated by strike and dip tic marks; long Northward, the Defiance monocline dashed line separates the Chaco slope from the Central Basin and is drawn approximately where the steps to the east to become the steeply dip- Upper Cretaceous Pictured Cliffs Sandstone is subaerially exposed (modified from Fassett, 1989). ping Hogback monocline west and northwest of Farmington (Fig. 3). The Hogback monocline has a complex history stretch- Northern and southern margins of the basin to the east and west along strike-slip faults. ing back to Paleozoic time, and seismic The Chaco slope is the seemingly passive A northeastward indentation stress array lines show a downward-flattening, west- structural transition between the Zuni is also suggested by the fault pattern dipping fault with both normal and uplift and the inner basin. It is an area of northeast of the uplift (Fig. 4). As much as reverse senses of motion depending on the relatively gentle homoclinal dip to the 5 km (3 mi) of left-lateral slip is suggested age of offset (Taylor and Huffman, 1988). north and northeast, but includes broad along escape faults on the western margin The present down-to-the-east offset of folds and significant fault zones. The Zuni of the Zuni uplift (Chamberlin and Ander- 1,200 m (3,937 ft) is commonly inferred to uplift has deceptively low topographic son, 1989), a rough indication of the order have occurred during eastward- to south- relief, but in fact there is over 4,000 m of magnitude of indentation. eastward-directed thrusting in Laramide (13,000 ft) of structural relief between the On the opposite side of the basin, the time. Huffman and Taylor (1999) suggest, crest of this northwest-striking asymmetric San Juan uplift, approximately 100 km (62 however, that this persistent zone of struc- structure and the deepest part of the San mi) in diameter, lies immediately north of tural weakness has also accommodated Juan Basin to the north (Kelley, 1956, 1963; the Hogback monocline. It has been right-lateral Laramide shear, similar to the Woodward and Callender, 1977). obscured by the superimposed Tertiary Defiance monocline, and this is our pre- The Zuni uplift has had a complex tec- San Juan volcanic complex. Kelley notes, ferred interpretation. tonic history, going back to late Paleozoic as far back as 1957, that the floor of the San The Hogback monocline bends north- time (Jentgen, 1977). Chamberlin and Juan Basin “is tilted northward and its eastward north of Farmington, where it is Anderson (1989) suggest that the present deepest part generally adjoins the greater interrupted by several re-entrant anticlines configuration of the Zuni uplift/Chaco uplift in the San Juan dome as though they and domes such as Ute dome and Barker slope complex is the result of Laramide were counterparts of a single mechanism dome. These small basement-block uplifts indentation-extrusion tectonics. As the at depth” (Kelley, 1957, p. 49), and we sug- suggest volume constraints during right- Zuni block was pushed northward and gest that there is a direct mechanical rela- lateral transpression along en echelon northeastward into the San Juan Basin, tionship between the two as there is basement faults (e.g., Ralser and Hart, large slivers of strata were shoved laterally between Laramide block-thrust uplifts and 1999; Hart et al., 1999).

February 2003, Volume 25, Number 1 NEW MEXICO GEOLOGY 5 To the east Huffman and Taylor (1999) present seismic cross sections that suggest inward-directed, sled-runner type thrust planes in front of the Hogback monocline along the Archuleta anticlinorium. How- ever, these structures may also be interpreted as flower structures along right-lateral wrench faults (Taylor and Huffman, 1999; Huffman and Taylor, 2001). South- ward along this eastern basin margin, the Nacimiento uplift was formed by a series of north-striking, tilted Precambrian blocks at the southeastern edge of the San Juan Basin, where 3,000 m (9,842 ft) of total structural relief separates the uplift from the adjacent basin. The Laramide history of the Nacimiento uplift has been interpreted in different ways. Early interpretations (Renick, 1931) were that the uplift was a westward overthrust. Further mapping suggested that an early phase of right-lateral offset of 3–5 km (2–3 mi) formed the north northwest-south southeast striking en echelon folds along the western margin of the uplift (Baltz, 1967; Woodward et al., 1972, 1992). Right- lateral offset along this eastern margin of the basin as great as 20–33 km (12–20.5 mi) is interpreted by Cather (1999). This early tectonic phase was followed by transpressive, right-lateral wrench faulting during later Laramide time, lead- ing to local overthrusting westward into the basin (Woodward et al., 1972; Baltz, 1978; Woodward, 1983, 1987). Pollock et al. (1998) and Erslev (2001) recently revived the interpretation that most of the offset along this fault front was west-directed thrust motion, and that strike-slip offset of Laramide age was minor. The uplift was reactivated during late Tertiary time in association with extensional faulting along the Rio Grande rift (Kelley, 1957; Wood- ward, 1987). There is a structural transition from the southern end of the Nacimiento uplift into the northeast-striking normal faults of the Rio Puerco fault zone. Slack and Campbell (1976) and Slack (1973) suggest this fault zone was the result of right-lateral wrench movement with an offset of less than 2.5 km (1.5 mi). The fault zone has been related to right-lateral Laramide deformation induced by north-northeast directed, regional horizontal compression (Slack and Campbell, 1976). Tectonic features summary The loosely connected segments of the Hogback are not the record of a single structural noose constricting the basin with radial, inward-directed thrusting FIGURE 4—A) Structure map of the San Juan Basin, contours drawn on the base of the Dakota Sand- around the edges of the basin. Rather, the stone with intervals of meters x 100 relative to mean sea level. Contours are extrapolated in the area Hogback monocline and Nacimiento fault of the Zuni Mountains due to erosion (modified after Thaden and Zech, 1986). Faults (shown in are the aligned expression of en echelon black) along the southern margin of the basin are oriented in an array that resembles an ideal slip- line field shown in (B) below it. Many of the mapped faults show evidence of strike-slip offset. Large basement fault segments that accommo- arrow indicates direction of indentation. B) Plane-strain slip-line field for an indenter into a finite dated some degree of right-lateral, strike- plastic medium. Arrow indicates direction of indentation (after Tapponnier and Molnar, 1976). The slip offset on the eastern and western mar- slip-line field will change with boundary conditions and indenter shape (Tapponnier and Molnar, gins of the basin, combined with south- 1976; Molnar and Tapponnier, 1977; Tapponnier et al., 1986). ward-vergent thrust faulting from the San

6 NEW MEXICO GEOLOGY February 2003, Volume 25, Number 1 FIGURE 5—Regional extension fractures striking approximately north northeast in early Tertiary rocks southwest of Cuba, New Mexico.

Juan uplift area at the northern margin of tion. Mesaverde sandstones are less well influenced by the same local structure that the basin. The alignment and connection of cemented, typically with authigenic clay created the local east-west trending these structures into a relatively continu- and calcite. This difference in original and canyon, but the north-south fracture pat- ous curvilinear feature that outlines the diagenetic composition resulted in signifi- tern is still present in the background. more stable basement underlying the Cen- cantly different mechanical properties (rel- A similar suite of approximately north- tral Basin is coincidental. atively ductile Mesaverde sandstones vs. south striking, bed-normal extension frac- relatively brittle Dakota sandstones), and tures occurs in the tilted Dakota and Outcrop fracture descriptions thus the two intervals have different frac- Mesaverde sandstones at the north rim of ture characteristics despite having been the basin. (All fracture strikes are present- The earliest fractures (based on crosscut- exposed to the same stress system. Frac- ed here with bedding rotated back to the ting relationships) seen in outcrops around tures in the immature Mesaverde sand- horizontal.) Condon (1988, 1997; pers. the basin typically strike from north-south stones typically formed as relatively long, comm. 1999) mapped fracture strikes to north northeast-south southwest. Sec- irregular extension fractures, whereas the around this northern edge of the basin, ondary, local fracture systems associated quartzitic Dakota sandstones contain more reporting that the average strike of the ear- with local structures are commonly super- numerous but shorter and more closely liest fractures ranges from north north- imposed onto the early fractures, and may spaced extension fractures. Many of the west-south southeast to north-south, vary- even be more numerous or dominant in Dakota intervals also contain conjugate ing regularly with stratigraphic and struc- some outcrops to the point where they shear-fracture pairs described in detail tural position. Finch (1994) reported obscure the early fracture set. Whereas the below. similar north northwest fracture orienta- secondary fracture sets are typically relat- tions and photo-lineaments in lower Ter- ed to the local structure, the earlier formed, Fractures at the northern and southern margins tiary strata exposed in the Animas River regional Laramide-age fractures consis- The oldest, through-going fracture pat- valley south of Durango. tently strike from north-south to north terns in Dakota and Mesaverde strata at Our field data at the northern rim of the northeast-south southwest throughout the the southern margin of the San Juan Basin San Juan Basin generally concur with these basin (Figs. 5 and 6). More complete out- most commonly have north northeast- earlier observations: i.e., we have meas- crop fracture descriptions are given in south southwest strikes that are consistent ured from north-south to north northeast- Lorenz and Cooper (2001). over tens of square miles (Gilkey, 1953; south southwest strikes of bed-normal, The fractured Mesaverde and Dakota Gilkey and Duschatko, 1953; Santos, 1966; extension fractures in both the Mesaverde reservoirs described here are typically very Maxwell, 1976; Kirk and Zech, 1984; Kirk and Dakota sandstones in this area. How- fine to fine-grained and reasonably well and Sullivan, 1987; Robertson, 1990a, ever, in addition to the extension fractures, sorted sandstones. Compositionally, the 1990b; Hallett et al., 1999; Fig. 7). Fractures two pairs of conjugate shear planes are Mesaverde sandstones are relatively to the northwest of the Zuni uplift have a also present in Dakota sandstones. The immature, whereas Dakota sandstones are more north-northwesterly strike. To the Dakota conjugate-fracture pairs have two mature to quartzitic. The Dakota sand- north, Point Lookout sandstones of the consistent orientations: 1) bed-normal, stones are well cemented with siliceous Mesaverde Group at Chaco Canyon with a bed-parallel acute-angle bisector cement except where bioturbation mixed National Historical Park contain younger (i.e., strike-slip offset), and 2) oblique to clays into the sandstones before cementa- fracture patterns that appear to have been bedding and dipping at shallow angles,

February 2003, Volume 25, Number 1 NEW MEXICO GEOLOGY 7 pervasive in the less steeply dipping Mesaverde and Tertiary strata approximately 10 km (6 mi) west of the uplift (Fig. 5), suggesting that this fracturing was not a product of flexure. Moreover, there is a pervasive fabric of closely spaced vertical extension fractures (as dense as 22 fractures/m) striking consistently north-south, parallel to the mountain front, in the granite core of the Nacimiento uplift itself (well exposed near San Gregorio Lake: see Fig. 6). Outcrops of Dakota sandstones along NM–84 and at Heron Lake, within the Archuleta anticlinorium on the northeastern corner of the basin, show that the north-south to north northeast-south southwest theme of regional vertical extension fractures extends to these areas as well. However, conjugate shear fractures indicating northeast-southwest compression are also present in the outcrops at the latter site.

Discussion The distinctive, thrust-oriented conjugate fractures at Heron Lake and east of Duran- go are a significant deviation from the more common north-south extension-fracture fabric. This northeast-southwest directed thrust geometry may fit with the late-stage changes in Laramide motion vectors for the Colorado Plateau area suggested by Bird (1998) among others, or they may represent local variations in stress orientation related to nearby westward- and southwestward-directed thrusting during transpressive wrench faulting. A northeasterly trending maximum horizontal compression is also suggested by three pairs of conjugate deformation bands found in Jurassic sandstones at the northern end of the Rio Puerco fault zone, south- FIGURE 6—Rose diagrams of shear and extension fracture orientations in outcrop. The north to west of the town of San Ysidro. These three north-northeast oriented extension fractures are shown in black. Diagrams are overlain on an index conjugate deformation-band sets are prob- map illustrating the structural features of the San Juan Basin. Areas of steep dip are shown in green; ably related to a late Laramide, local stress CC = Chaco Canyon, SR = Shiprock, UD = Ute dome, CG = Campground, A Mtn = Animas Moun- tain, AR = Animas River, HL = Heron Lake, GR = Ghost Ranch, SGL = San Gregorio Lake, SY = San array created in the southeastern corner of Ysidro (base map modified after Fassett, 1989). the basin between the Zuni uplift and the Nacimiento uplift as described above. with a bed-parallel, acute-angle bisector areas these are the dominant fractures, They document a three-stage increase in (reverse dip-slip/thrust offset). The acute- whereas in other areas (Fig. 8) they are the magnitude of the northeast-southwest angle bisectors of both sets strike either poorly developed and their presence is oriented horizontal compressive stress, as northeast-southwest or north-south when obscured by younger fractures. it increased to become the intermediate bedding is rotated back to horizontal. The Cretaceous strata adjacent to the Pre- compressive stress and finally the maxi- relative age relationship between the con- cambrian-cored Nacimiento block, across mum compressive stress, with no rotation jugate fractures and the north-south exten- the basin on its southeastern side, have in orientation (Olsson et al., in press). sion fractures is obscure. been caught up in the steeply dipping Sandstones of the overlying Cretaceous cuestas that front the western face of this Dakota interval at the same San Ysidro Fractures at the eastern and western basin uplift. These strata contain high-strain location contain a significantly different margins deformation structures such as folds and pattern of fractures, consisting of an appar- Multiple sets of fractures are present in faults, as well as fractures created by the ent conjugate shear fracture pair with Mesaverde strata north and northwest of local bulldozing effects of the overthrust strikes of 10–20° and 60–70°. Intersection Farmington. Again, the oldest fracture set uplift. However, Baltz (1967, pp. 62–63) relationships, however, suggest that the along the Hogback and in the adjacent, reports that the most conspicuous frac- fractures striking 10–20° are an older set, less-deformed strata is a set of bed-normal tures strike 008–025° in this area, and that imposed on the rock as extension fractures extension fractures that strikes north these fractures imprint a north northeast- under the same conditions of north north- northeast-south southwest. Condon (1988) south southwest trending grain onto the east-south southwest maximum horizontal reports a similar average strike of 15° for topography. Low-altitude, oblique aerial stress seen widely elsewhere in the basin. these fractures between the Colorado– photos show that this generally north This set was present as a background fab- New Mexico border and Durango. In some northeast-south southwest pattern is also ric, and was reactivated in shear during

8 NEW MEXICO GEOLOGY February 2003, Volume 25, Number 1 FIGURE 7—Pervasive north northeast to northeast striking natural frac- McKinley County, New Mexico. Structure contours are drawn on the base tures within the Cretaceous Point Lookout Sandstone of the Mesaverde of the Dakota Sandstone; contour interval is 100 ft (modified from Kirk Group illustrated on the geologic map of the Dalton Pass quadrangle, and Sullivan, 1987). See Figures 4 and 6 for location in the basin. formation of the younger, 60–70° shear Fractures in core These cores were examined in order to fractures, as it had the optimum strike with document the subsurface fracture charac- respect to the re-oriented stresses to Cores from 19 wells drilled into Mesaverde teristics, to compare them with outcrop become one set of the conjugate pair. The and Dakota strata and stored at the New fracture characteristics, and to provide complementary shear-fracture set of the Mexico Library of Subsurface Data in insights into the viability of extrapolating pair formed ab initio. Socorro were studied for this project. outcrop fracture data into the subsurface. None of the cores examined for this study were oriented. Natural fractures in the Mesaverde sandstones are primarily vertical extension fractures filled or partially filled with calcite and/or quartz. Quartz may in fact be an early mineralization phase that is obscured under later calcite layers in most fractures. Most of the cored fractures are less than 1 mm in total width, and the nar- rower fractures are typically completely occluded by mineralization. Striated bedding-parallel shear fractures are also present locally in Mesaverde sandstones. Similar but more common vertical extension fractures occur in cores of the underlying Dakota sandstones, although vertical, intersecting conjugate fractures may also be present in this layer (Lorenz et al., 1999). Dakota fractures are filled or partially filled with quartz, calcite, and locally with kaolinite in successive layers or patches. Horizontal stylolites are common FIGURE 8—Plan-view fracture map on the Hogback monocline east of the town of Shiprock, New in the cleaner Dakota sandstones, and Mexico. Note the through-going, oldest, north-south striking fracture set. short, wide fractures having variable ori-

February 2003, Volume 25, Number 1 NEW MEXICO GEOLOGY 9 dynamics of the San Juan Basin within its Laramide orogenic setting. The model can be used iteratively to estimate Laramide stress orientations across the basin, to pre- dict the probable orientations of natural fractures that formed under such conditions, and to understand observed fracture patterns in outcrops and the subsurface. We suggest that most of the strain recorded by the pervasive fracturing of Cretaceous strata in the San Juan Basin was caused by the displacement of the San Juan and Zuni uplifts toward each other, with the San Juan Basin caught between them. The kinematics were ultimately driven by continental-scale, base-of-the- crust crustal subduction, which jostled the basement blocks in the crust below the San Juan Basin against each another along ancient faults. Jostling raised some blocks, such as the San Juan and Zuni uplifts, which were then thrust laterally into the adjacent basin, affecting stresses and creating fractures in the shallower strata. The stress orientations within the sedimentary units filling the basin were the result of the frontal configuration and indentation vectors of these uplifts, and were not directly related to the more regional plate motions and geometries. Orientations of the pre- existing faults, more than the orientation of the deep crustal stresses or directions of subduction, dictated the geometry of the FIGURE 9—Map illustrating the northeastward yielding of the Col- resulting structures. In general the result- orado Plateau relative to the Cordilleran foldbelt, the Rocky Moun- ant stress trajectory was north-south to tain foreland, and the San Juan Basin (modified after Woodward north northeast-south southwest within and Callender, 1977). the San Juan Basin, as recorded by the fracture sets within the Dakota Sandstone and entations and filled with kaolinite may pronounced in Dakota reservoirs because Mesaverde Group. extend a few tens of centimeters vertically of a higher degree of fracturing, but tests Concurrent with indentation from the from the larger teeth of the stylolites. measuring the anisotropy for this unit north and south, basement blocks on either The core data suggest that there is a set have not been reported. side of the Hogback monocline and Naci- of sub-parallel fractures, striking parallel miento uplift were transpressively wrench to the maximum in situ horizontal com- Tectonic model faulted against each other with a right-lat- pressive stress (as indicated by associated eral sense of motion. Deep-seated Lara- petal fractures: Lorenz et al., 1990) in the The Laramide orogeny, extending from lat- mide strain was accommodated by base- Mesaverde and Dakota subsurface strata. est Cretaceous through Eocene time, was ment thrusting at the northern and south- The orientation of this fracture set is not the first tectonic event to influence the San ern basin margins, and by basement restricted by the data presented here, but Juan Basin following deposition of the wrenching at the eastern and western mar- published fracture orientations for subsur- Dakota and Mesaverde strata under study. gins. face core (Ortega and Marrett, 2000; Reconstructions suggest that the Colorado TerBest, 1997; Lorenz et al., 1999) suggest Plateau, which encompasses the San Juan Discussion that the same north northeast-south south- Basin within its southeastern corner, was Although right-lateral shear was concen- west fracture fabric seen in outcrop is also translated northeastward with respect to trated at the basin margins, small amounts dominant in the subsurface. The east-west the Rocky Mountain foreland during the of similar offset probably occurred along fracture indications reported by Ortega Laramide orogeny (Fig. 9; Kelley, 1957; the northeast-southwest and northwest- and Marrett (2000) for the Sunray H Comp Woodward and Callender, 1977; Chapin southeast trending basement faults within #6 well may be the product of an anom- and Cather, 1981; Woodward et al., 1992). the basin at this time as well, enhancing alous stress orientation near the north- In the process, the Colorado Plateau may fracturing in the immediately overlying south striking Bonito dike (Fig. 1). have been rotated several degrees clock- strata. Evidence for additional, more per- This north northeast-south southwest wise about a pole in northern Texas vasive shear within the Cretaceous strata is fracture orientation is consistent with pro- (Hamilton, 1981, 1988). Many northeast- suggested by conjugate shear fractures duction engineering data that show a hor- facing Laramide folds within the Cordiller- found in Dakota cores (Lorenz et al., 1999). izontal permeability anisotropy, on the an foldbelt of southwestern New Mexico Asymmetry between the northern and order of 10:1, elongated in the north-south (Corbitt and Woodward, 1973) document southern indenters may have contributed to north northeast-south southwest direc- related northeastward yielding. to this shear, as the positions of these struc- tion in Mesaverde reservoirs in many parts The descriptions of the individual structures and their directions of thrusting are of the basin (e.g., Harstad et al., 1998; New tural features and their kinematics present- not directly opposed to each other (Fig. Mexico Institute of Mining and Technolo- ed earlier (pages 5–6) can be synthesized 10). Differential motion of the Colorado gy, 2000). Such anisotropy should be less into a conceptual model of the tectonic Plateau (i.e., faster northeastward move-

10 NEW MEXICO GEOLOGY February 2003, Volume 25, Number 1 ment at the western edge of the basin than at the eastern side: see diagrams of Bird, 1998) also might have contributed to pervasive right-lateral shear. As described above, the resulting com- pressional and shear strains in the strata of the basin are recorded by north-south to north northeast-south southwest striking vertical extension fractures within the Mesaverde Group and Dakota Sandstone along with conjugate fractures in the Dakota. High horizontal stresses, coupled with overpressuring in Eocene to Oligocene time (Bond, 1984), would have produced stress conditions and mechanical properties favorable to fracturing. Most of the observed fracture characteristics and orientations are compatible with this north-south to north northeast-south southwest shortening within the San Juan Basin. In fact, the two pairs of conjugate shear fractures in Dakota strata at the northern edge of the basin (described earlier, page 8) suggest exceptionally strong north-south compression. One conjugate pair has a bed-parallel axis of intersection, and the other has a bed-normal axis of intersection, but both have bed-parallel, acute-angle bisectors that strike approximately north-south. These conjugate, strike-slip, and thrust-oriented fracture pairs are consistent with a southward- directed horizontal compressive force, one that exceeded the magnitude of the overburden stress for a time at these locations. It is commonly assumed (although not published) that the north-south striking extension fractures in the San Juan Basin formed under conditions of east-west Rio Grande extension. However, extension does not produce strike-slip or reverse- FIGURE 10—Tectonic fracture model of the San Juan Basin. A dominant oldest set of ver- dip-slip conjugate patterns. Only signifi- tical extension fractures striking primarily north northeast-south southwest is observed cant compression forms such conjugate across the basin. These features are primarily the result of southward and northward geometries, and only the Laramide oroge- indentation of the San Juan and Zuni uplifts, respectively (base map after Kelley, 1957). ny produced compression of any sort in the region of the San Juan Basin during post-Jurassic time. Moreover, it is unlikely sion in parts of the basin include a change Archuleta dike swarm in the northeastern that the strata hosting north-south exten- from wrench faulting to overthrusting corner of the basin (Fig. 1), represent a sion fractures would have remained along the Nacimiento front (Woodward et response to Rio Grande rift extension. unfractured during the high-stress, north- al., 1972), resulting in compression of the Post-Laramide extension fractures with south Laramide compressive conditions strata between that front and northeast- north-south strikes could conceivably have that formed the observed conjugate frac- ward indentation of the Zuni uplift. formed in the Cretaceous strata in this ture systems. It is plausible, and more like- Laramide shortening was replaced by extensional setting. However, earlier frac- ly, that the extension and conjugate frac- incipient Basin and Range extension in turing is the preferred interpretation ture systems are genetically related to the west-central New Mexico approximately because extension can not account for the same Laramide north-south thrust events, 36 million years ago (Cather, 1989), and conjugate fracture pairs, many examples of a correlation that is strongly supported by extensional structures related to the Rio shear, or for the north-south thrusting seen the common symmetry axes of the two Grande rift have overprinted parts of the in the outcrop and subsurface at several fracture systems. San Juan Basin. The east-west Basin and scales. Moreover, extensional faulting of Other conjugate shear fractures Range extensional tectonic system has axes the crust would concentrate fractures over (described earlier, page 8) suggest that of symmetry that are, coincidently, nearly the normal faults rather than producing locally, as at Heron Lake in the northeast parallel to the shallow Laramide stress the pervasive, dilational fracture texture corner of the basin and near San Ysidro in axes. However, whereas Laramide tecton- observed. Baltz (1967, 1978) in fact sug- the southeastern corner of the basin, the ics were derived from an increase in the gested that there is little or no evidence for maximum horizontal compressive stress north-south horizontal compressive stress, late Tertiary east-west extension across was oriented northeast-southwest. These the later extensile regime involved a northern New Mexico, other than the nor- shear fractures may be related to a late decrease in the east-west horizontal com- mal faults that accommodate the rift itself Laramide stress superimposed locally onto pressive stress. Some large-scale paleo- and the north-south striking igneous strata in the corners of the basin. Possible stress indicators, such as the post- dikes. sources of northeast-southwest compres- Laramide (Miocene), north-south trending It is not the intent of this paper to con-

February 2003, Volume 25, Number 1 NEW MEXICO GEOLOGY 11 tribute to the discussion of the different compressive stress does not appear to be Walck for constructive comments and models currently under consideration for widespread within the basin. reviews, which have enhanced this manu- Laramide kinematics in the southern These subsurface fracture and stress pat- script. Sandia is a multiprogram laboratory Rocky Mountain region (e.g., Cather, 1999; terns control a significant horizontal per- operated by Sandia Corporation, a Lock- Erslev, 2001; Bird, 1998; Yin and Ingersol, meability anisotropy in the Cretaceous heed Martin Company, for the United 1997). Rather, our intent is to integrate the reservoirs of the San Juan Basin, and sup- States Department of Energy under con- aspects of those different models that are port the tectonic model presented earlier. tract DE-AC04-94AL85000. compatible with fracturing in the San Juan The orientation of this stress anisotropy Basin, recognizing that the fractures that has not been significantly modified by the References record Laramide stresses in the shallow younger east-west extensional tectonics reservoir rocks do not necessarily reflect associated with Rio Grande rifting. Baltz, E. H., 1967, Stratigraphy and regional tecton- the contemporaneous basement-level ic implications of part of Upper Cretaceous and stresses and kinematics. At the simplest Tertiary rocks, east-central San Juan Basin, New Summary Mexico: U.S. Geological Survey, Professional level, our fracture data support dilation Paper 552, 101 pp. and fracturing of the Cretaceous strata The conceptual model constructed here for Baltz, E. H., 1978, Resume of Rio Grande depres- from compression along a north-south to the tectonic setting in which Cretaceous sion in north-central New Mexico; in Hawley, J. north northeast-south southwest axis, reservoirs in the San Juan Basin fractured W. (comp.), Guidebook to Rio Grande rift in New with evidence for compression along a suggests that an approximately north- Mexico and Colorado: New Mexico Bureau of northeast-southwest axis at certain locali- Mines and Mineral Resources, Circular 163, pp. south directed compressive stress created 210–228. ties. fractures in the shallow reservoir strata Bird, P., 1998, Kinematic history of the Laramide during the Laramide orogeny. This stress orogeny in latitudes 35º–49ºN, western United Subsurface stresses system resulted indirectly from northeast- States: Tectonics, v. 17, pp. 780–801. ward translation of the San Juan Basin as Bond, W. A., 1984, Application of Lopatin’s method Stress is important in that 1) it creates and part of the Colorado Plateau, during which to determine burial history, evolution of the geot- hermal gradient, and timing of hydrocarbon gen- orients natural fractures, 2) it dictates, the basin was caught between southward along with mechanical properties, the eration in Cretaceous source rocks in the San Juan indentation of the San Juan uplift and Basin, northwestern New Mexico and southwest- behavior of fractures during hydrocarbon north- to northeastward indentation of the ern Colorado; in Woodward, J., Meissner, F. F., production (Lorenz, 1999), and 3) it dic- Zuni uplift. Contemporaneous, right-later- and Clayton, J. L. (eds.), Hydrocarbon source tates the orientation and behavior of al wrench motion was concentrated along rocks of the greater Rocky Mountain region: hydraulic stimulation fractures. However, the Nacimiento uplift and Hogback mono- Rocky Mountain Association of Geologists, Guidebook, pp. 433–447. few measurements documenting the in cline, defining the margins of the inner situ stress orientations within the shallow Cather, S. M., 1989, Post-Laramide tectonic and vol- basin. These shallow, basin-scale stresses canic transition in west-central New Mexico; in reservoirs of the San Juan Basin have been were controlled by local basement-seated Anderson, O. J., Lucas, S. G., Love, D. W., and made, and fewer published. The large, structures and thus were the indirect prod- Cather, S. M. (eds.), Southeastern Colorado north-south striking Tertiary dikes of the uct of, but not co-linear with, continental- Plateau: New Mexico Geological Society, Guide- Archuleta dike swarm (Fig. 1), clustered in scale, Laramide crustal dynamics. book 40, pp. 91–97. the northeastern part of the basin and scat- Cather, S. M., 1999, Implications of Jurassic, Creta- Within this framework, a set of vertical ceous, and Proterozoic piercing lines for tered sparsely elsewhere, suggest that the extension fractures, striking generally from Laramide oblique-slip faulting in New Mexico maximum in situ horizontal stress was ori- north-south to north northeast-south and rotation of the Colorado Plateau: Geological ented north-south during the time of southwest (but with local variations), was Society of America, Bulletin, v. 111, no. 6, pp. intrusion (Miocene). This does not con- formed in both Mesaverde and Dakota 849–868. Chamberlin, R. M., and Anderson, O. J., 1989, The strain either the stress orientations at the sandstones. Additional sets of conjugate significantly earlier time of fracturing, or Laramide Zuni uplift, southeastern Colorado shear fractures were created in some of the Plateau—A microcosm of Eurasian-style indenta- the current stress conditions. Dakota strata. Fractures in the immature tion-extrusion tectonics?; in Anderson, O. J., Present-day subsurface stress-orienta- Mesaverde sandstones typically formed as Lucas, S. G., Love, D. W., and Cather, S. M. (eds.), tion indicators include coring-induced relatively long, irregular extension frac- Southeastern Colorado Plateau: New Mexico petal fractures in oriented core, and well- tures, whereas the quartzitic Dakota sand- Geological Society, Guidebook 40, pp. 81–90. Chapin, C. E., and Cather, S. M., 1981, Eocene tec- bore breakouts seen in oriented caliper or stones contain short, sub-parallel, closely wellbore-image logs. Lorenz et al. (1999), tonics and sedimentation in the Colorado spaced extension fractures. These fractures Plateau–Rocky Mountain area; in Dickinson, W. and unpublished company reports, sug- are a primary control on hydrocarbon pro- R., and Payne, M. D. (eds.), Relations of tectonics gest that these indicators in the San Juan duction within the Cretaceous-age tight- to ore deposits in the southern Cordillera: Ari- Basin show a maximum horizontal com- gas sandstone reservoirs of the San Juan zona Geological Society, Digest, v. 14, pp. pressive stress striking consistently from Basin. 173–198. north-south to north northeast-south Condon, S. M., 1988, Joint patterns on the northwest side of the San Juan Basin (Southern Ute southwest across most of the interior Acknowledgments Indian Reservation), southwest Colorado; in Fas- basin, consistent with the engineering per- sett, J. E. (ed.), Geology and coal-bed methane meability-anisotropy data. A wellbore- This work has been supported by the resources of the northern San Juan Basin, Col- image log run recently by Marathon near National Energy Technology Laboratory, orado and New Mexico: Rocky Mountain Associ- the Hogback northwest of Farmington Morgantown, West Virginia, as part of the ation of Geologists, Guidebook, pp. 61–68. recorded a 035° maximum horizontal Geotechnology for Low-Permeability Condon, S. M., 1997, Fracture studies on the Upper Cretaceous Pictured Cliffs Sandstone and Fruit- stress orientation (J. Bucci, pers. comm. Reservoirs contract to Sandia National land Formation, northern San Juan Basin, La 2000). Laboratories. Our thanks to Tom Mroz, Plata County, Colorado; in Close, J. C., and Casey, Yale et al. (1993) reported a nearly north- NETL contract manager, and to Jim T. A. (eds.), Natural fracture systems in the southeast-southwest (41°) trend for the maxi- Ammer, project manager for the San Juan ern Rockies: Four Corners Geological Society, mum horizontal compressive stress in four Basin study, for their support. We would Guidebook, pp. 85–96. wells in an unspecified field in the south- also like to thank the New Mexico Library Corbitt, L. L., and Woodward, L. A., 1973, Tectonic framework of Cordilleran foldbelt in southwest- eastern corner of the basin. This probably of Subsurface Data (New Mexico Bureau of ern New Mexico: American Association of Petro- reflects a late Laramide influence of the Geology and Mineral Resources) for access leum Geologists, Bulletin, v. 57, no. 11, pp. nearby Nacimiento uplift. The northeast- to core. Our gratitude to Steve Cather, Eric 2207–2216. southwest trending maximum horizontal Erslev, William Olsson, and Marianne DuChene, H. R., 1989, Fracture reservoirs in the San

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February 2003, Volume 25, Number 1 NEW MEXICO GEOLOGY 13 quadrangle, McKinley and Valencia Counties, San Juan Basin and vicinity, New Mexico and Geologists, Guidebook, pp. 27–42. New Mexico, U.S. Geological Survey, Geologic Colorado: U.S. Geological Survey, Geologic Woodward, L. A., 1983, Potential oil and gas traps Quadrangle Map GQ-517, scale 1:24,000. Investigations Series I-2641, scale 1:500,000. along the overhang of the Nacimiento uplift, Slack, P. B., 1973, Structural geology of the north- TerBest, H., Jr., 1997, Open apertures, fracture ori- northwestern New Mexico; in Lowell, J. D., and east part of the Rio Puerco fault zone, Sandoval entations, and their correlation to production in Gries, R. (eds.), Rocky Mountain foreland basins County, New Mexico: Unpublished M.S. thesis, the Mesaverde Group in the San Juan Basin and uplifts: Rocky Mountain Association of Geol- University of New Mexico, 74 pp. (abs.); in Close, J. C., and Casey, T. A. (eds.), Nat- ogists, Guidebook, pp. 213–218. Slack, P. B., and Campbell, J. A., 1976, Structural ural fracture systems in the southern Rockies: Woodward, L. A., 1987, Geology and mineral geology of the Rio Puerco fault zone and its rela- Four Corners Geological Society, Guidebook, p. resources of Sierra Nacimiento and vicinity, New tionship to central New Mexico tectonics; in 147. Mexico: New Mexico Bureau of Mines and Min- Woodward, L. A., and Northrop, S. A. (eds.), Tec- Thaden, R. E., and Zech, R. S., 1986, Structure con- eral Resources, Memoir 42, 84 pp. tonics and mineral resources of southwestern tour map of the San Juan Basin and vicinity; in Woodward, L. A., and Callender, J. F., 1977, Tecton- North America: New Mexico Geological Society, Turner-Peterson, C. E., Santos, E. S., and Fishman, ic framework of the San Juan Basin; in Fassett, J. Special Publication 6, pp. 46–52. N. S. (eds.), A basin analysis case study—Morri- E. (ed.), San Juan Basin III: New Mexico Geologi- Steven, T. A., 1975, Middle Tertiary volcanic field in son Formation, Grants uranium region, New cal Society, Guidebook 28, pp. 209–212. the southern Rocky Mountains—Cenozoic histo- Mexico: American Association of Petroleum Woodward, L. A., Hultgren, M. C., Crouse, D. L., ry of the southern Rocky Mountains: Geological Geologists, Studies in Geology, no. 22, pp. 35–45. and Merrick, M. A., 1992, Geometry of Nacimien- Society of America, Memoir 144, pp. 75–93. Tremain, C. M., Laubach, S. E., and Whitehead, N. to–Gallina fault system, northern New Mexico; in Tapponnier, P., and Molnar, P., 1976, Slip line field H., III, 1991, Coal fracture (cleat) patterns in Lucas, S. G., Kues, B. S., Williamson, T. E., and theory and large-scale continental tectonics: Upper Cretaceous Fruitland Formation, San Juan Hunt, A. P. (eds.), San Juan Basin IV: New Mexi- Nature, v. 264, pp. 319–324. Basin, Colorado and New Mexico—implications co Geological Society, Guidebook 43, pp. 103–108. Tapponnier, P., Peltzer, G., and Armijo, R., 1986, On for coalbed methane exploration and develop- Woodward, L. A., Kaufman, W. H., and Anderson, mechanics of the collision between India and ment; in Schwochow, S. D., Murray, D. K., and J. B., 1972, Nacimiento fault and related struc- Asia; in Coward, M. P., and Ries, A. C. (eds.), Col- Fahy, M. F. (eds.), Coalbed methane of western tures, northern New Mexico: Geological Society lision tectonics: Geological Society of London, North America: Rocky Mountain Association of of America, Bulletin, v. 83, no. 8, pp. 2383–2396. Special Publication 19, pp. 115–157. Geologists, Guidebook, pp. 49–59. Yale, D. P., Strubhar, M. K., and El Rabaa, A. W., Taylor, D. J., and Huffman, A. C., Jr., 1988, Over- Weijermars, R., 1997, Principles of rock mechanics, 1993, Determination of hydraulic fracture direc- thrusting in the northwestern San Juan Basin, Alboran Science Publishing, Lectures in geo- tion, San Juan Basin: Society of Petroleum Engi- New Mexico–A new interpretation of the Hog- science, 360 pp. neers, Production Operations Symposium, paper back monocline (abs.): U.S. Geological Survey, Whitehead, N. H., III, 1997, Fractures at the surface, number SPE 25466, pp. 543–553. Research on Energy Resources, Programs with San Juan Basin, New Mexico and Colorado; in Yin, A., and Ingersoll, R. V., 1997, A model for evo- Abstracts, pp. 60–61. Hoak, T. E., Klawitter, A. L., and Blomquist, P. K. lution of Laramide axial basins in the southern Taylor, D. J., and Huffman, A. C., Jr., 1998, Map (eds.), Fractured reservoirs—characterization Rocky Mountains, U.S.A: International Geology showing inferred and mapped basement faults, and modeling: Rocky Mountain Association of Review, v. 39, no. 12, pp. 1113–1123.

14 NEW MEXICO GEOLOGY February 2003, Volume 25, Number 1 Gallery of Geology Tierra Amarilla anticline

This photo, looking north, shows the Tierra Amarilla anticline, deposit cascading tongues of light-colored travertine along the located 4 mi southwest of the village of San Ysidro, New Mexico. north (not seen in this photo) and west sides of the core. The light- The feature is surely a fine example of an anticline and probably colored horizontal strip in the upper right of the photo is the the most accessible anticline in New Mexico. stream bed of the Rio Salado, and just beyond it (but not visible) The sharp flanks of this Laramide-age structure are held up by is US–550 and the southern tip of the Nacimiento uplift. the light-colored Todilto Formation overlying the Entrada Sand- The southern two-thirds or so of the anticline is on U.S. Bureau stone, both of Middle Jurassic age. The soft underlying slopes are of Land Management (BLM) land (secs. 21 and 28 T15N R1E). The made up of the Upper Triassic Petrified Forest Formation of the northern one-third is on New Mexico state land (sec. 16 T15N Chinle Group. R1E). Access to the BLM part is from a parking spot along The raised core of the structure in the valley is underlain by a Cabezon Road on the south. Access to the northern part is either welt of highly contorted Petrified Forest mudstones and channel from the same southerly route with a longer walk or from the sandstones. The core is capped by a carapace of Pleistocene and north over a tricky four-wheel-drive road that crosses a parcel of Holocene travertine. The older travertine reaches a maximum land recently acquired by Zia Pueblo. One needs to purchase a thickness of about 10 m (33 ft). This competent material slopes permit to set boot on the state-owned land. away from the linear crest, has rifted along an axial fault system, and is gliding and breaking up downslope over the incompetent —Dirk Van Hart Petrified Forest mudstones. Some springs are still active and Photo taken on May 25, 2001

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